Current aircraft fuel monitoring and indicating systems measure the fuel height and, separately, its density to calculate the weight of the fuel and, therefore, the remaining fuel capacity present in a fuel tank. In some such systems, for example, seven wired sensors, e.g. capacitive sensors, are variously placed about the bottom surface of the fuel tank—i.e. so that they are covered by the fuel in the tank. A predetermined profile is used to account for irregularities in the tank configuration/shape. These systems require that electrical current-carrying wires connected to the capacitive sensors be located within each fuel tank, with the consequent possibility of a spark that can ignite the fuel vapors and contained fuel and cause an in-flight explosion or fire, as is believed to have occurred in the crash of TWA Flight 800. Periodic inspection or prophylactic, periodically-scheduled replacement of such wiring located within the fuel tanks is both expensive and largely impractical for complex aircraft which are in virtually constant operation, requiring that the fuel tanks be fully drained of costly remaining fuel and the aircraft removed from service for an extended interval.
In recognition of the catastrophic effects of electrical sparking within an aircraft fuel tank containing fumes emitted by the fuel, and recognizing the impracticality of requiring regular inspection or replacement of such in-tank wiring, the U.S. Federal Aviation Administration (FAA) has recently mandated that commercial airlines pump nitrogen into aircraft fuel tanks to fill those volumetric portions of the tanks not containing fuel and thereby minimize the risk of such explosions and/or fire. This modification, however, provides but a limited remedy that, at most, reduces only the severity of the problem since the most likely existing sources of electrical sparks—i.e. electrical wiring associated with in-tank fuel sensors—nevertheless remain present within the fuel tanks of commercial aircraft.